Metal pipes are sometimes subjected to cold drawing in order to improve the dimensional accuracy thereof or smooth the inner and outer surfaces thereof. For cold drawing, a die and a plug are generally used. The die reduces the diameter of a metal pipe to adjust the outside diameter of the metal pipe to a desired dimension. Further, the die smoothes the outer surface of the metal pipe. On the other hand, the plug adjusts the inside diameter dimension of the metal pipe, and smoothes the inner surface of the metal pipe.
In recent years, for the plug for use in cold drawing, various types of geometry according to purposes have been proposed. For example, JP2006-167763A and JP11-300411A disclose protruding ring plugs which are intended to remove the wrinkle flaw on the inner surface of a metal pipe, the workpiece, (hereinafter to be called a hollow shell). With the protruding ring plugs disclosed in these Patent Documents, a ring-shaped projection is formed in the rear portion of the plug, the projection providing a step height on the plug surface. By the use of this step height, the hollow shell inner surface is subjected to ironing. Thereby, the wrinkle flaw on the hollow shell inner surface is removed and the surface roughness is improved.
In cold drawing, the outer surface of the metal pipe after the drawing often has tensile residual stresses in the circumferential direction. If the outer surface of the metal pipe has a dent flaw, the mutual action between the dent flaw and the tensile residual stress may cause a crack to occur on the outer surface of the metal pipe at the time of heat treatment which is carried out after the cold working. Therefore, it is preferable that the tensile residual stress on the outer surface of the metal pipe after the cold drawing be the smallest possible.
JP2-197313A discloses a method for manufacturing of metal pipes that reduces the residual stress and improves the internal pressure fatigue characteristics of the metal pipe. The method of this Patent Document adopts a plug of a two-step construction, in which the rear half portion of the plug has an outside diameter larger than that of the front half portion of the plug. Using this plug, the hollow shell whose diameter has been reduced by the die is expanded at a expansion ratio of 0.1 to 1.5%. Thereby, the residual stress in the metal pipe after the cold drawing is changed, the internal pressure fatigue characteristics being improved.
However, if the expansion ratio is only defined as 0.1 to 1.5% as disclosed in JP2-197313A, the tensile residual stress on the outer surface of the metal pipe may not be able to be reduced, although the residual stress in the metal pipe is changed.